Particularly in hydraulic brake systems for motor vehicles in which the hydraulic connections have to transmit pressures of 70 bar and more, threaded or screw connections using male screws or nuts, as are known in principle from German standards DIN 74233 and 74234, represent the most widely used prior art. During both assembly on the production line and disassembly of these connections, a change has meanwhile taken place from using a spanner to using an electric or pneumatic screwdriver, so that the time required to produce a hydraulically sealed connection could in this way be reduced. However, particularly during assembly of these connections in the overhead position, the problem still exists that the internally and externally threaded sections of the connecting parts may be placed on one another in a manner tilted with respect to one another, before the connection is tightened by means of the screwdriver. The latter switches off when a predefined tightening torque is reached, said predefined tightening torque indicating a hydraulically sealed connection when the threaded connection has been correctly made, but merely feigning a hydraulically sealed connection in the case of a tightening torque which is increased on account of tilting of the connecting parts. The resulting lack of sealing of the connection is often discovered only when the hydraulic system is filled, whereupon cost-intensive remedial measures have to be taken. In the worst case scenario, the lack of sealing is discovered only upon operation of the hydraulic system, resulting in highly dangerous situations.
Besides the conventional threaded or screw connections, releasable plug-in connections for pipelines are also known in the prior art (cf. for example DE 35 31 926 C2 or DE 102 90 508 T1). In this connection, the generic document DE 296 10 434 U1 by the Applicant discloses an end piece of a hydraulic pipe on which a flange is formed, around which a plastic abutment is injection-molded in a form-fitting manner. The abutment has a flat annular face remote from the pipe end, said annular face extending perpendicular to the central axis of the hydraulic pipe. The end piece can be plugged into a connecting hole in an insertion sleeve of a releasable plug-in connection and is secured in the latter by means of a securing element made of round spring steel wire in order to prevent it from being pulled out, wherein the annular face of the abutment can be supported on the securing element in the axial direction of the hydraulic pipe. The abutment here furthermore has a cylindrical section which is delimited by the annular face at its side remote from the pipe end and surrounds the flange of the hydraulic pipe. The cylindrical section, which serves the purpose of guiding the end piece into the connecting hole of the insertion sleeve, in particular with respect to an elastic sealing element which is arranged in the connecting hole, is adjoined in a step-free manner in the direction of the pipe end by a conical section which tapers in the direction of the pipe end from the diameter of the cylindrical section to approximately the external diameter of the hydraulic pipe and serves to elastically expand the securing element radially outwards when the plug-in connection is joined together, before the securing element latches behind the annular face of the abutment at the end of the joining operation, after passing the cylindrical section.
Although such releasable plug-in connections can be assembled overhead more easily compared to the conventional threaded or screw connections and are just as hydraulically resistant to high pressures—as long as they are correctly assembled—and thus are in principle also suitable for brake applications in motor vehicles, they are still subject to reservations in the automotive industry since, during manufacture on the production line, it is not yet possible to reliably ascertain, with reasonable outlay, whether or not the hydraulic connection has been correctly joined.